The present invention relates generally to a picture distribution service system, and more particularly to a cable television picture distribution service system which allows many subscribers to receive various kinds of distributed picture services, such as cable television programs, by making efficient use of a limited number of transmission channels accommodated in the cable television picture distribution service system.
Recently, the Broadcast aspects of Integrated Services Digital Network (B-ISDN) have been developed as a broadband transit switching network for giving many subscribers a faster data transmission service and a finer picture transmission service. It is expected that the transmission speed used in the broadband transit switching netwrok will be around 150 Mbps, and the use of a transmission speed four times as fast as 150 Mbps, or 600 Mbps, is being studied in the development phase. This broadband transit switching network will enable a faster, greater capacity transmission of picture information services and cable television programs to many subscribers through the B-ISDN.
During recent years, the application of a cable television distribution system to the broadband transit switching network has been studied. In this cable television distribution system, an optical fiber cable as a transit line linking a central switch to a number of remote terminals and to a subscriber line linking each remote terminal to subscriber terminals has been used for achieving a faster, greater capacity data transmission in the broadband transit switching network. For example, a conventional broadband transit switching network system used for a telephone switching system is illustrated in FIG. 1. This conventional broadband transit switching network system generally has a central switch CO, a number of remote equipment units RE, and a number of subscriber terminals 4A, 4B, 4C, . . . which may be, for example, visual telephone terminals. As shown in FIG. 1, a transit looped line L made of an optical fiber cable is capable of data transmission at a rate of 2.4 Gbps in a so-called time slot process and is used for linking the central switch CO to the plurality of remote equipment units RE. A number of subscriber lines "l" each made of an optical fiber cable are capable of data transmission at a rate of 600 Mbps and these lines are used for linking the remote equipment units RE to the subscriber terminals. The central switch CO is a station in which a switching is carried out. The remote equipment unit RE which comprises a concentrating part S1, a separating part S2, a signal processing unit SGU and a call processing unit CPU is provided for carrying out a concentration/separation of subscriber lines from the plurality of subscriber terminals. A concentrated or separated line from the remote equipment units RE is connected to the transit looped line L. The central switch CO comprises a switch SW and a call processing unit (CPU).
In the conventional broadband transit switching system employing an optical fiber cable as shown in FIG. 1, there still exists some unused data transmission capacity even when a number of visual telephone terminals are used as the subscriber terminals in the broadband transit switching system. Thus, there is a demand to make efficient use of this broadband transit switching system. And, there is a problem in that, when the same picture information source is called from too many subscriber terminals through the central switch at the same time, network congestion takes place due to the limited number of transit lines in the broadband transit switching system. In such a case, it is impossible for the conventional system to supply the picture information to all the subscribers who have called the picture information and therefore some of them experience difficulties in receiving the picture information normally distributed from the center switch.